Improve Django admin navigation for backup data

Add linked admin summaries for hosts, snapshots, and backup runs so the SQL-first
backup state is easier to inspect from the Django admin. Hosts now link to their
filtered snapshot and run lists, backup runs link back to their snapshot, and
snapshots show base/run relationships without requiring filesystem inspection.

Cover the new admin display helpers with focused tests.

src/pobsync_backend/admin.py

@@ -1,6 +1,10 @@
 from __future__ import annotations
 
 from django.contrib import admin
+from django.db.models import Count
+from django.urls import reverse
+from django.utils.html import format_html
+from django.utils.http import urlencode
 
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord
 
@@ -33,7 +37,16 @@ class GlobalConfigAdmin(admin.ModelAdmin):
 
 @admin.register(HostConfig)
 class HostConfigAdmin(admin.ModelAdmin):
-    list_display = ("host", "address", "enabled", "updated_at")
+    list_display = (
+        "host",
+        "address",
+        "enabled",
+        "schedule_state",
+        "snapshot_count_link",
+        "backup_run_count_link",
+        "latest_run_state",
+        "updated_at",
+    )
     list_filter = ("enabled",)
     search_fields = ("host", "address")
     readonly_fields = ("created_at", "updated_at")
@@ -47,18 +60,66 @@ class HostConfigAdmin(admin.ModelAdmin):
         ("Timestamps", {"fields": ("created_at", "updated_at"), "classes": ("collapse",)}),
     )
 
+    def get_queryset(self, request):
+        return super().get_queryset(request).annotate(
+            snapshot_count=Count("snapshots", distinct=True),
+            backup_run_count=Count("runs", distinct=True),
+        )
+
+    @admin.display(description="Schedule")
+    def schedule_state(self, obj: HostConfig) -> str:
+        try:
+            schedule = obj.schedule
+        except ScheduleConfig.DoesNotExist:
+            return "none"
+        if not schedule.enabled:
+            return "disabled"
+        return schedule.cron_expr
+
+    @admin.display(description="Snapshots", ordering="snapshot_count")
+    def snapshot_count_link(self, obj: HostConfig) -> str:
+        count = getattr(obj, "snapshot_count", None)
+        if count is None:
+            count = obj.snapshots.count()
+        url = _admin_changelist_url("pobsync_backend", "snapshotrecord", {"host__id__exact": obj.pk})
+        return format_html('<a href="{}">{}</a>', url, count)
+
+    @admin.display(description="Runs", ordering="backup_run_count")
+    def backup_run_count_link(self, obj: HostConfig) -> str:
+        count = getattr(obj, "backup_run_count", None)
+        if count is None:
+            count = obj.runs.count()
+        url = _admin_changelist_url("pobsync_backend", "backuprun", {"host__id__exact": obj.pk})
+        return format_html('<a href="{}">{}</a>', url, count)
+
+    @admin.display(description="Latest run")
+    def latest_run_state(self, obj: HostConfig) -> str:
+        latest = obj.runs.order_by("-created_at").first()
+        if latest is None:
+            return "none"
+        return f"{latest.status} {latest.started_at:%Y-%m-%d %H:%M}" if latest.started_at else latest.status
+
 
 @admin.register(BackupRun)
 class BackupRunAdmin(admin.ModelAdmin):
-    list_display = ("host", "run_type", "status", "started_at", "ended_at", "snapshot")
+    list_display = ("host", "run_type", "status", "started_at", "ended_at", "snapshot_link")
     list_filter = ("run_type", "status", "started_at")
     search_fields = ("host__host", "snapshot_path", "snapshot__dirname", "snapshot__path")
     autocomplete_fields = ("snapshot",)
+    list_select_related = ("host", "snapshot")
+    date_hierarchy = "started_at"
+
+    @admin.display(description="Snapshot", ordering="snapshot__dirname")
+    def snapshot_link(self, obj: BackupRun) -> str:
+        if obj.snapshot is None:
+            return ""
+        url = reverse("admin:pobsync_backend_snapshotrecord_change", args=[obj.snapshot.pk])
+        return format_html('<a href="{}">{}</a>', url, obj.snapshot.dirname)
 
 
 @admin.register(SnapshotRecord)
 class SnapshotRecordAdmin(admin.ModelAdmin):
-    list_display = ("host", "kind", "dirname", "status", "base", "started_at", "discovered_at")
+    list_display = ("host", "kind", "dirname", "status", "base_link", "backup_run_count_link", "started_at", "discovered_at")
     list_filter = ("kind", "status", "base_kind", "started_at", "discovered_at")
     search_fields = (
         "host__host",
@@ -69,11 +130,38 @@ class SnapshotRecordAdmin(admin.ModelAdmin):
         "base_snapshot_id",
     )
     autocomplete_fields = ("base",)
+    list_select_related = ("host", "base")
     readonly_fields = ("discovered_at",)
 
+    def get_queryset(self, request):
+        return super().get_queryset(request).annotate(backup_run_count=Count("backup_runs"))
+
+    @admin.display(description="Base", ordering="base__dirname")
+    def base_link(self, obj: SnapshotRecord) -> str:
+        if obj.base is not None:
+            url = reverse("admin:pobsync_backend_snapshotrecord_change", args=[obj.base.pk])
+            return format_html('<a href="{}">{}</a>', url, obj.base.dirname)
+        if obj.base_dirname:
+            return obj.base_dirname
+        return ""
+
+    @admin.display(description="Runs", ordering="backup_run_count")
+    def backup_run_count_link(self, obj: SnapshotRecord) -> str:
+        count = getattr(obj, "backup_run_count", None)
+        if count is None:
+            count = obj.backup_runs.count()
+        url = _admin_changelist_url("pobsync_backend", "backuprun", {"snapshot__id__exact": obj.pk})
+        return format_html('<a href="{}">{}</a>', url, count)
+
 
 @admin.register(ScheduleConfig)
 class ScheduleConfigAdmin(admin.ModelAdmin):
     list_display = ("host", "cron_expr", "enabled", "prune", "last_status", "last_started_at", "updated_at")
     list_filter = ("enabled", "prune", "last_status")
     search_fields = ("host__host", "cron_expr")
+    list_select_related = ("host",)
+
+
+def _admin_changelist_url(app_label: str, model_name: str, params: dict[str, object]) -> str:
+    base_url = reverse(f"admin:{app_label}_{model_name}_changelist")
+    return f"{base_url}?{urlencode(params)}"